The present invention relates to metering liquid coating material flow in a voltage block type color change system which delivers coating material to an electrostatic spraying apparatus.
Color changers for spray coating apparatus have application in industrial operations where articles are to be spray coated at a station or as they move along a production line. Where the articles are to be coated with a wide variety of colors, it generally is not practical to establish separate spray stations or production lines for each color, or even to spray a long sequence of articles one color, then another long sequence a second color, etc. Instead, it is desirable to be able to make color changes rapidly and simply at a single station.
Electrostatic spray coating devices have an increased painting efficiency over nonelectrostatic types. When painting with an electrostatic spraying apparatus, it is necessary to have some means for applying a charge to the paint. In some apparatus, charging is accomplished by an electrode connected to a high voltage supply and placed in close proximity to or in contact with the paint either just prior or close to its point of atomization. In rotary atomization apparatus, the rotary atomizer is ordinarily made of a conductive material and connected to the power supply, so the atomizer itself is the electrode. Whichever type of apparatus is used, the charging potential usually is on the order of several tens of kilovolts. The electrostatic charging process works well when spraying nonconductive paints, but when spraying conductive paints, such as waterborne paints, precautions must be taken to prevent the high voltage at the spraying apparatus from shorting to ground through the conductive column of paint being delivered to the spraying apparatus.
One known approach to prevent shorting the high voltage to ground is to isolate the entire paint supply and color change system from ground potential. This allows the paint system to "float" at the charging potential, but has the drawback that a large amount of electrical energy is capacitively stored in the system. To prevent the capacitively stored energy from presenting a shock hazard to operating personnel, it is necessary to provide a protective enclosure around the color changer and paint supplies, which increases costs and requires that the spraying operation be shut down and the system electrically discharged whenever it is necessary to replenish the supplies of paint. Also, the large amount of capacitively stored energy increases the probability of arcing and the possibility of an explosion when volatile materials are sprayed.
Another approach that can be used with relatively nonconductive paint, is to ground the paint supplies and color changer and connect the spraying apparatus to the color changer through a hose of sufficient length that the electrical resistance of the paint column in the hose is large enough to reduce current leakage through the paint column to a level that does not short out the charging voltage or cause it to fall to an unacceptably low level. A disadvantage of the technique is that a long hose is difficult to clean during color changes. Also, while the extended length of the hose limits the magnitude of leakage current, some leakage nonetheless occurs and represents "wasted" charging energy. For relatively conductive coating materials such as waterborne paints, the resistance of the paint is so low that the technique is not practical.
A more recent approach to electrostatically spraying waterborne paints contemplates utilizing a voltage block, which is some means for interrupting the electrically conductive path that would otherwise exist between an electrostatic spray gun and a grounded paint supply, in order to spray waterborne paint electrostatically without grounding out the high electrostatic charging voltage. An exemplary embodiment of such a voltage block is disclosed in Michael J. Diana U.S. Pat. No. 4,932,589, assigned to the assignee of the present invention. Said Diana patent discloses an apparatus for supplying conductive paint from a grounded color changer to an electrostatic spray gun while electrically isolating the color changer from a high voltage at the gun. The apparatus has a reservoir into which a metered quantity of paint to be sprayed is introduced. After filling the reservoir with paint and prior to delivering the paint to the spray gun, the flow path between the reservoir and color changer is cleaned to electrically isolate the reservoir and paint therein from the grounded color changer. The paint is then delivered from the reservoir to the spray gun for being emitted in an electrostatically charged atomized spray. At the end of spraying, the high voltage is turned off and the reservoir is cleaned and reloaded with another color of paint. To minimize the time required between color changes, the apparatus advantageously has two reservoirs that alternately receive and deliver selected colors of paint to the spray gun, such that one reservoir is cleaned and reloaded with paint while the other delivers paint to the spray gun.
Another voltage block type color change system in which reservoirs are used is disclosed in Elberson et al. U.S. Pat. No. 4,792,092.
In the voltage block type color change systems disclosed in said Diana and Elberson et al. patents, a metered quantity of paint is flowed into a reservoir for delivery to an electrostatic spray gun. However, neither patent discloses any means for delivering substantially all of the paint in the reservoir to the spray gun while ensuring that the paint supply is not fully depleted, which could result in the introduction of air into the spray gun and "spitting" of paint from the gun. Also, neither patent teaches any means for ensuring delivery of paint from the reservoir to the spray gun at an accurately controlled volume flow rate. Consequently, in each of the systems, increased costs for paint are incurred since it often happens that an excess quantity of paint remains behind at the end of a spraying operation, and in neither is there precise control over the deposition rate of paint on an article.